To observe change of a global environment, pH of sample solutions having high concentration of salts obtained from sea level or sea abyss in the ocean and plain water sample solutions obtained from rivers, lakes or marshes on land are measured. The measurement of pH is conducted on site or at a laboratory after obtaining thereof.
Recently, carbon dioxide included in the atmosphere has been gradually increased because of a large amount of artificial consumption of fossil fuels such as petroleum, coal and the like. Global warming has progressed. Carbon dioxide in the atmosphere has increased concentrations of hydrogen carbonate ions and carbonate ions by dissolving in marine water, land water, cloud and rainwater and thus, acidification of the ocean and so on has increased. By continuously and accurately measuring pH of marine water of the ocean, lake water of a brackish lakes and plain water of rivers, lakes or marshes and so on, change of the global environment is observed and a future environment on a global scale is forecasted. To take actions for preventing further environmental pollution and global warming, accurate pH data should be obtained.
An available and generalized pH meter is used so as to measure pH of ordinary analyte solutions such as tap water, dilute acid or alkaline test fluid and sample at room temperature maintained at a constant temperature of approximately 25° C. According to the pH meter, pH of the analyte solution can be measured from an electrochemical voltage between a glass electrode and a reference electrode. In this case, pH of the analyte solution is expressed as a hydrogen-ion exponent (pH=−log [H+]; [H+] represents a hydrogen-ion concentration in a solution). Actually, hydrogen-ion activity rather than the hydrogen-ion concentration itself is electrochemically measured. Because the hydrogen-ion activity approximately equals to the hydrogen-ion concentration in the ordinary analyte solution, pH thereof may be represented by the hydrogen-ion exponent.
It is difficult to accurately detect the hydrogen-ion concentration itself and the hydrogen-ion activity in the sample solution having high concentration of salts such as marine water, because of existence of co-existence ions with high concentration, interaction between the co-existence ions and the hydrogen ions, ion selectivity of electrodes, action of dissolved gases and the like. Therefore it is also difficult to rigorously measure pH of the sample solution having high concentration of salts.
Besides the hydrogen-ion exponent used in pH of the ordinary analyte solution, to represent pH of marine water, definitions of total scale pH (hereinafter referred to it as pHT. pHT=−log([H+]+[HSO4−]); [H+] represents a hydrogen-ion concentration in the solution, and [HSO4−] represents a sulfate ion concentration therein.) and seawater scale pH (hereinafter referred to it as pHsws. pHsws=−log([H+]+[HSO4−]+[F−]); [H+] represents a hydrogen-ion concentration in the solution, [HSO4−] represents a sulfate ion concentration therein and [F−] represents a fluorine ion concentration therein.) are disclosed by Non Patent Document 1. The total scale pH and the seawater scale pH are premised on measuring pH of marine water, and can be defined by conducting calibration of both of the glass electrode and the reference electrode. The calibration is conducted by using a calibration solution for measuring marine water (Tris-HCl buffer solution, AMP buffer solution and so on) which is prepared by dissolving a buffer into a solvent having composition similar to marine water or the same. Preparing of the calibration solution for measuring marine water requires complex steps and complicated works. In addition, the calibration solution for measuring marine water on site is not suitable for measuring pH of the analyte solution of collecting marine water of the ocean whose liquid temperature varies wide range from 0 to 35° C., because the pH thereof varies widely depending on a temperature just like one of the Tris-HCl buffer solution.
As a method for simply measuring pH of an analyte solution with extreme precision, Patent Document 1 discloses a method of measuring pH of a measurement solution as marine water by using a pair of electrodes consisting of a glass electrode and a reference electrode. A potential difference between the pair of the electrodes is regulated to 0 mV in a solution having range of pH 7.2 to 8.2. The pH of the measurement solution is measured according to a voltage generated between the electrodes.
According to the method, because the potential difference between the electrodes is needed to set to 0 mV, an internal liquid of the glass electrode have to be regulated to pH 7.2 to 8.2 by a concentration of saturated potassium chloride. When the measurement solution is marine water, the internal liquid employing a pH standard solution that approaches actual pH thereof is preferably used. According to a study by the present inventor, when potassium chloride was saturated in the pH standard solution, it has become clear that pH of the internal liquid was decreased by approximately 0.5 from original pH of the pH standard solution due to variation in an activity coefficient. A set condition in the method disclosed by Patent Document 1 therefore requires employing a pH standard solution having pH 7.7 to 8.7 as the internal liquid which meets a requirement of pH 7.2 to 8.2. As available pH standard solutions, JIS (Japanese Industrial Standard) buffer solutions includes a phthalate buffer solution of pH 4.01, a neutral phosphate buffer solution of pH 6.86, a phosphate buffer solution of pH 7.41 and a borate buffer solution of pH 9.17 at 25° C. as a constant temperature. But a pH standard solution which is stable in pH 7.7 to 8.7 is not available in a market. Further, unless original pH based on constituents of a pH buffer solution of internal liquids in the glass electrode and the reference electrode is carefully selected, errors thereof occur.
An aqueous solution consisting potassium chloride from 3.3 mol/L to saturation or an aqueous solution, in which potassium chloride and the JIS buffer solution that approaches pH of a sample solution such as marine water are co-existed, has been used for internal liquids of the glass electrode and the reference electrode. When measuring pH of an analyte solution such as marine water, pH thereof has been calculated from the voltage between the electrodes by using original pH of a pH standard solution as a basis without considering the concentration of potassium chloride and a liquid temperature in the internal liquid of the reference electrode.
When carbon dioxide included in the atmosphere dissolves in marine water, it generates hydrogen carbonate ions (HCO3−) and carbonate ions (CO32−), and then these ions neutralize acid (H+) and buffer it. Thereby pH of marine water approaches neutrality (approximately pH 7.4 to 8.2). To observe change of the global environment accurately, a variation of pH should be accurately and precisely detected down to 3 decimal places. A temperature of marine water in the ocean widely varies range from approximately 0 to 35° C. depending on a region and/or depth. However, the variation of pH, which is caused by the concentration of potassium chloride in the internal liquid and a liquid temperature thereof in the glass electrode and the reference electrode, has not been reflected. The errors between the actual pH and the above calculated pH therefore have occurred. Furthermore, the variation of pH has not able to be accurately and precisely detected down to 3 decimal at measurement places on site.